A fossil fuel such as gasoline or light oil is burned in the internal combustion engine of a vehicle. The exhaust gas generated by burning of the fossil fuel contains environmental pollutants such as unburned carbon monoxide (CO), carbon hydride (HC), sulfur oxide (SOx), nitrogen oxide (NOx) and so forth, as well as water and carbon dioxide. In the recent years, various countermeasures to purify vehicle exhaust gas have been adopted particularly in order to protect the environment and prevent pollution of living environments.
One such countermeasure is the use of an exhaust gas purifying catalyst device. In this countermeasure, a three-way catalyst for purifying exhaust gas is placed midway in the exhaust system, where CO, HC, NOx and the like are decomposed by oxidation-reduction reaction to make them harmless. In order to ensure continuous decomposition of NOx in the catalyst device, a urea aqueous solution is sprayed over the catalyst from immediately upstream of the catalyst device in the exhaust system.
The urea aqueous solution is contained in a urea aqueous solution tank mounted in a vehicle and its concentration may change over time and in the tank, uneven concentration distribution may occur locally. Since the urea aqueous solution being supplied from the tank through a supply pipe to a spray nozzle by a pump is generally taken through the tank outlet near the tank bottom, for enhancement of the efficiency of the catalyst device it is important that the urea aqueous solution in this area has prescribed urea concentration.
It may actually happen that a liquid other than a urea aqueous solution is mistakenly contained in the urea aqueous solution tank. In that case, it is necessary to detect quickly that the liquid is not a urea aqueous solution with prescribed urea concentration and give a warning, in order for the catalyst device to fulfill its function.
For this reason, a fluid state identification device is used to determine whether or not the aqueous solution is a urea aqueous solution whereof urea concentration is within a prescribed range. An example of this type of fluid state identification device is described in JPA 2007-263950 and some such devices have an identification sensor part including a thermal sensor, such as particularly an indirectly-heated sensor (indirectly-heated liquid type detector).
JPA 2007-263950 points out that deterioration in the identification performance of a liquid identification device is caused by the presence of air bubbles in the measured liquid, and describes that the surface of the identification sensor is covered by a hydrophilic film so as to prevent adhesion of air bubbles and improve the identification performance. JPA 2007-263950 also points out that deterioration in the identification performance of the liquid identification device is caused by forced flow of the measured liquid, and describes that a flow control plate surrounding a thermal sensor is provided in the identification sensor part to improve the identification performance.
JPA 2005-84025 points out that deterioration in the identification performance of a liquid identification device with an identification sensor part including a thermal sensor is caused by forced flow of the measured liquid due to vibration of the tank, etc. and describes that the identification sensor part is surrounded by a covering body and a hole for circulation is made in the covering body so as to improve the identification performance.
JPA 2011-218670 describes that an identification sensor part including a thermal sensor is formed by molding and it is covered by a covering body and a hole through which a liquid passes is made in the covering body.
JPA 2011-242227 describes that a drip tube is located opposite to a sensing part in order to avoid the influence of foam diffusion due to dripping of a urea aqueous solution.